Devices for monitoring the performance of internal combustion engines have proliferated in recent years as the need for greater performance, fuel economy and pollution control with such engines has increased. For years a wide variety of transducers have been used for measuring water temperature, oil pressure and temperature, manifold vacuum and engine speed. Numerous other engine parameters, such as timing, dwell angle, and other characteristics have been detected through measurements taken from the engine electrical system. However, performance monitoring systems developed thus far have largely been limited to use in measuring parameters which are detectable through the drive train or from external or bolt-on equipment such as the distributor, the carburetor, the vacuum, fuel and cooling system lines, the radiator or the electrical system. For numerous performance monitoring applications, and in particular for engine diagnostic instruments it is highly desirable to obtain an indication of the performance of the engine internally. The present invention satisfies this need for internal performance monitoring in that it provides for measurements of the pressure developed within the cylinder itself in an economical and universally applicable manner.
Diesel engines, in particular, pose problems from a performance monitoring or diagnostic standpoint in that the conventional spark distribution system, a convenient source of timing and speed signals, is either not present or is rendered inoperative during normal operation. Monitoring devices relying on speed-related signals, such as a tachometer, assume an added degree of complication and expense in the absence of an electrical distributor system. While signals representing engine speed can be taken from transducers on the flywheel or dynamic damper, the infinite number of damper gear ratios found on different engine models has impeded the development of a truly universal flywheel pick-up system. Speed-related signals have also been taken from engine alternators, but the output on such devices is dependent on the voltage regulator and requires amplification for low charging conditions. Speed-related signals can also be developed from the belts, pulleys, timing chains and other mechanical engine components, but most of these devices are either inaccessible from a practical standpoint or vary with the make and model of the engine and as such do not lend themselves to universal application with monitoring equipment.
Systems have been devised by others which utilize the sequential performance of the fuel-injection pump or lines for developing signals useful in engine timing. For example, U.S. Pat. No. 3,511,088 of Preston R. Weaver discloses a system wherein a piezo-electric quartz pressure transducer is adapted for coupling to a fuel line so as to provide an output signal which varies with the radial expansion resulting from increased pressure within the fuel line. While the arrangement disclosed in the aforesaid patent may provide a general indication of changes in line pressure, it is of no value in analysis of the combustion process within the cylinder itself in that the fuel line is isolated from the combustion chamber or cylinder much of the time by pressure regulating springs and valves within the injector nozzle assembly. In addition, the aforesaid system requires various electrical and mechanical damping components for reducing the effects of engine and fuel line vibration on the transducer output signal.
Piezoelectric transducers have also been used in laboratory-type cylinder pressure guages wherein the quartz material is housed in a special transducer fitting which is tapped through the engine housing into the proximity of the walls of the cylinder combustion chamber. Devices of this type are described in "Diesel and High Compression Gas Engines" 3rd Ed. 1974 by Kates and Luck, Chapter 17. However, such devices rely on stress detected through the cylinder walls and require either specially casted engine housings or engine housing modifications which make them impractical for widespread use as diagnostic devices outside the laboratory.